This invention relates to an internal combustion engine system including an exhaust gas recirculation circuit, wherein each combustion chamber of the engine is provided with a plurality of spark plugs.
In the attempt of minimizing the contribution to the air pollution of exhaust gas of internal combustion engines, particularly automotive engines, it is not easy to satisfactorily lower the concentration of nitrogen oxides (NOx) in the exhaust gas. Conventional methods of minimizing the emission of NOx are classified roughly into two groups: one group intends catalytic reduction of NOx within engine exhaust systems, and the other group consists of various methods for the suppression of the formation of NOx in engine combustion chambers by repressing the maximum combustion temperature, more particularly, by retarding the ignition timing, operating the engine with a lean air-fuel mixture or recirculating a portion of exhaust gas into the combustion chambers.
The use of reduction catalysts leads to a noticeable increase in engine system manufacturing costs and accompanies the need of replacing the catalysts at certain intervals. The second group methods are estimated to be more economical than the catalytic reduction methods and considered to principally be advantageous. However, the repression of the maximum combustion temperature tends to result in incompleteness or instability of the combustion. In other words, the formation of NOx is suppressed by the second group methods usually with the sacrifice of fuel economy (specific fuel consumption) and/or output characteristic of the engines. The prevention of NOx emission, indeed, must be accomplished. However, also it is important to maintain or even improve the high-level performance, including fuel economy, of automotive internal combustion engines hitherto developed. A proper balance should be maintained between the suppression of the formation of NOx and the engine performance.
Among the aforementioned methods for the suppression of the formation of NOx, the recirculation of a portion of exhaust gas to the combustion chambers has the advantage that the fuel economy and output characteristic are not significantly sacrificed in this method so long as the quantity of the recirculated exhaust gas is appropriate. However, it has been recognized that the combustion in the engine becomes unstable, causing a significant degradation of the engine performance, if the exhaust gas recirculation rate (defined as the volumetric ratio of the recirculated exhaust gas to air drawn into the combustion chambers) exceeds a certain level. It is practically impossible to raise the exhaust gas recirculation rate above about 10% for conventional engines. On the other hand, the suppression of the formation of NOx does not reach maximal at the exhaust recirculation rate of 10%. The formation of NOx can more strongly be suppressed by greatly raising the recirculation rate beyond 10%.
Recently attention has been paid by researchers of Nissan Motor Co., Ltd. to the fact that the formation of NOx can be suppressed to a fully satisfactory extent practically without the sacrifice of fuel economy or output characteristic of the engine with the maintenance of a stable engine operation when the exhaust gas recirculation is accomplished at considerably high recirculation rates only if the combustion at each engine cycle is completed in an appropriately shortened period of time. A new engine system recently proposed on the basis of this fact is characterized by the employment of an exhaust recirculation rate far greater than 10% (when maximized: the recirculation rate is controlled according to the operational condition of the engine) and the provision of two or more spark plugs for each combustion chamber of the engine. The plural spark plugs in this engine are arranged so as to individually share approximately equally divided volumes of each combustion chamber and actuated substantially simultaneously. Accordingly flame propagation in each combustion chamber starts at two or more distant ignition points and is completed with a greatly shortened propagation distance. As a result, a greatly diluted air-fuel mixture can stably and completely be burned with a relatively low maximum of the combustion temperature.
The provision of two spark plugs for a single combustion chamber per se is an old technique for improving the reliability of the ignition but has been considered disadvantageous for the suppression of NOx formation because of contributing to a rise in the maximum combustion temperature. The above referred new engine system is unique in that a plurality of spark plugs are provided for each combustion chamber when a great drop of the combustion temperature is intended by recirculating the exhaust gas in large quantities. This engine system fundamentally attained a success in balancing the prevention of the NOx emission with the maintenance of a good engine performance by rapidly completing a low temperature combustion.
However, this new engine system will not attain a practical success unless various factors such as the number and arrangement of the spark plugs, the exhaust recirculation rate and the configuration of the combustion chambers are particularly determined in correlation with each other.